for ArcGIS Desktop (Arc Map and ArcCatalog) you can start with the tutorials in the help
Get started with ArcMap—Help | ArcGIS Desktop
For ArcGIS Online, online classes, you may want to look at Esri MOOC - Massive Open Online Course
Take advantage of all the free recorded technical sessions Esri Videos | E360 Videos: GIS, Events, ArcGIS Products & Industries And free and paid options https://www.esri.com/training/
those se are a few sources to get you started, but to get more specific info, you should include a bit more information about what software you have access to and you goal.
Here's a direct link to all Esri free training resources for ArcMap (includes web courses, seminars, lessons, etc.): http://www.esri.com/training/Bookmark/B12FdP00x
Retrieving data ...